转载地址，做了部分补充。谢谢原作者。
[tr?ns?leit]
vi.翻译, 能被译出
vt.翻译, 解释, 转化, 转变为, 调动
在这里用到的意思是转化，转变,转换，在linux下输入tr --help查看一下提示：
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amosli@amosli-pc:~$tr--help
Usage:tr[OPTION]... SET1 [SET2]
Translate, squeeze, and/ordelete characters from standard input,
writing to standard output.
-c, -C, --complement use the complement of SET1
-d, --delete delete charactersinSET1,donot translate
-s, --squeeze-repeats replace each input sequence of a repeated character
that is listedinSET1 with a single occurrence
of that character
-t, --truncate-set1 first truncate SET1 to length of SET2
--help display this help andexit
--version output version information andexit
SETs are specified asstringsof characters. Most represent themselves.
Interpreted sequences are:
\NNN character with octal value NNN (1 to 3 octal digits)
\\ backslash
\a audible BEL
\b backspace
\f form feed
\n new line
\rreturn
\t horizontal tab
\vvertical tab
CHAR1-CHAR2 all characters from CHAR1 to CHAR2inascending order
[CHAR*]inSET2, copies of CHARuntillength of SET1
[CHAR*REPEAT] REPEAT copies of CHAR, REPEAT octalifstarting with 0
[:alnum:] all letters and digits
[:alpha:] all letters
[:blank:] all horizontal whitespace
[:cntrl:] all control characters
[:digit:] all digits
[:graph:] all printable characters, not including space
[:lower:] all lowercaseletters
print all the element (key) in the list after the given operations. two consequtive keys should be separated by a single space.。replace unsafe characters with their escape sequences. unsafe characters are those characters that may be altered during transport across the internet, and include the (<, >, ", #, {, }, |, \, ^, ~, [, ], and ') characters. this flag applies to all urls, including opaque urls.。replace unsafe characters with their escape sequences. unsafe characters are those characters that may be altered during transport across the internet, and include the (<, >, ", #, {, }, |, \, ^, [, ], and ') characters. this flag applies to all urls, including opaque urls.。
